Polyesters comprising terephthalic acid and ethylene glycol residues and residues of one or more saturated dicarboxylic acids comprise a growing market of specialty polyesters. The incorporation of a saturated comonomer such as 1,4-cyclohexanedicarboxylic acid (CHDA) into the polyethylene terephthalate (PET) framework can impart desirable properties, e.g., slower crystallization rates, to the resulting polyester. Copolyesters normally are prepared by reaction of a purified dicarboxylic acid such as CHDA and diols under polymerizing conditions. Coployesters derived from CHDA or a diester therof are significantly more expensive due to the cost of the CHDA monomer. For example, 1,4-CHDA normally is prepared commercially from terephthalic acid (TPA) by the ruthenium-catalyzed hydrogenation of the disodium salt of TPA, followed by neutralization and separation of the resulting CHDA from aqueous sodium salts. The present invention provides a process for the production of copolyesters containing CHDA residues by the hydrogenation of polyester oligomers containing TPA residues.
Hydrogenation processes of involving terephthalic acid esters derived from poly(ethylene terephthalate) are known. U.S. Pat. No. 3,501,420 discloses the depolymerization of waste poly(ethylene terephthalate) in an alcohol to give a solution of terephthalic acid esters. The resulting solution is hydrogenated to remove color bodies. In this process, poly(ethylene terephthalate) is converted to primarily monomeric esters before contact with hydrogen. The decolorized solution of TPA esters may be used to prepare high-quality poly(ethylene terephthalate).
U.S. Pat. No. 3,487,100 discloses that bis-hydroxyethyl terephthalate (BHET) prepared from crude TPA and ethylene glycol may be decolorized by treatment with hydrogen in the presence of a hydrogenation catalyst. The BHET is dissolved in water and filtered to remove oligomeric species (which are insoluble in water). The filtered solution is treated with hydrogen at a temperature range of 50–100° C. Although the hydrogenation treatment reduces fluorenone impurities, the BHET must be in solution and the product must be crystallized from solution before further use. Another disadvantage is that polyester oligomers cannot be processed by this method.
Japanese Kokai JP 50-142537 discloses a process to produce cyclohexanedimethanol (CHDM) by the depolymerization and hydrogenation of waste poly(ethylene terephthalate). In this process, waste PET is depolymerized with a 7-fold excess of ethylene glycol in the presence of hydrogen and a hydrogenation catalyst to convert poly(ethylene terephthalate) into a solution of ethylene glycol esters of cyclohexanedicarboxylic acid. The product of this step is separated from the catalyst and treated again with hydrogen at high pressure in the presence of a second hydrogenation catalyst. The object of the second step is to convert the esters of cyclohexanedicarboxylic acid into a solution of CHDM.
U.S. Pat. No. 6,380,352 discloses a process to remove colored impurities from oligomers produced from crude terephthalic acid and ethylene glycol by passing the oligomers over a hydrogenation catalyst. These oligomers have a degree of polymerization of 2 to 20 (preferably 3 to 5) and b* value of 4 to 7. The hydrogenation catalysts are composed of the platinum group metal supported on zirconia, or carbon, or silica, or alumina, or zeolites, or their mixtures. The process is preferably operated at 240 to 270° C. and 150 to 400 psig.